Black Spots on SBT-10 Geiger Muller Tube

Archived from radmon.org - originally posted 19/03/2017

A radmon.org user posted a question asking what the black spots were in his SBT-10. I had recently bought one of these from a seller in Ukraine and mine too has the black spots similar to the one posted on radmon.org. The seller assured me that mine was NOS (new-old-stock), meaning whilst it is old, it has merely been stored for the time and never used. It appears to be working correctly regardless of it's condition. I tested each element separately and each reads around the same CPM as the others so I'm assuming that whilst cosmetically nasty, the black spots appear to do no harm. What causes them I'm unsure of, but I suspect it is some kind of age related issue. Possibly some kind of reaction between the materials and the gasses used in constructing these tubes. I wish I could help you more. I too would like to know what the spots are and if they are impacting the reliability of the tube.

On a side note, I have found a person in Lithuania that supplies a 3D printed connector with pins or just the pins themselves here: http://arduino-geiger-pcb.blogspot.co.uk/2015/03/a-new-contact-socket-for-sbt-10-10.html

After some close inspection I sort of know what the black spots are now. They are some kind of crystalline chemical or mineral that is growing on the surface of the mica.

I put my SBT-10A under my stereo microscope a few weeks ago (before I had a camera for my microscopes) and could see they were hexagonal in shape. Some were like triangles with their points cut off. They appear as arbitrary shapes and almost like little blobs and that is because some grow next to others and some grow on top of others. They appear to be very very thin, although there were a couple that I could just see had a little more length to them. I think they are being caused by some chemical in the resin leaching out and reforming as crystals.

I thought I would follow up with some microscope images of them, but I managed to destroy my tube by accident by dropping a polarizing filter from my microscope onto the tube. File Attachment: So that's the second SBT-10A that has broken on me. File Attachment:

Oh well, at least we can see inside one now! I'll do a separate post on that.